The potential utilization of zinc electrodes in rechargeable alkaline electrochemical cells, in particular nickel-zinc batteries, has long been realised [1]. Nickel-zinc batteries have been demonstrated to have good performance criteria, including:
i) energy density of 55-85 Wh/kg,
ii) power density of 140-200 W/kg,
iii) open circuit potential of 1.75 V, and
iv) a self discharge rate of <0.8%/day.
However, a number of problems associated with the zinc electrode exist that prevent such batteries being charged and discharged for sufficient cycles to be of practical use. These problems arise from the propensity of the zinc electrode to exhibit changes in shape, commonly referred to as ‘shape change’, upon discharge/charge cycling. Shape change is caused by the solubility of the zinc electrode discharge products in the alkaline electrolyte as the species zincate, Zn(OH)42−. Eventually, solid zinc hydroxide does precipitate out of solution onto the electrode surface, but this may occur at locations remote from the discharge reaction site. Consequently, upon recharging, the zinc electrode becomes thicker in some locations and thinner in others. This may occur on a gross scale, where electrodes swell or fracture respectively, or may be limited to decreasing the active surface area of zinc metal crystallites on the surface of the zinc electrodes.
In addition to shape change, the phenomenon of zinc dendrite growth also takes place. Dendrite growth occurs when redeposition of zinc metal on recharge takes place at a collection of points on the electrode surface, rather than as an even distribution across the entire surface. The source of zinc material for reduction is not the precipitated zinc hydroxide, but the dissolved zinc in solution. Narrow needles or dendrites of zinc metal grow from the electrode surface and eventually form an internal electrical short to the nickel electrode causing the battery to fail.
A number of different approaches have been attempted to negate or prevent the effects of shape change and dendrite growth. These are, for the most part, focused on decreasing the solubility of the zinc electrode discharge products in the electrolyte. This has been attempted by modifications to the zinc electrode active mass, electrolyte and separators, the latter to prevent growth of dendrites through the separator which otherwise would short the cell. These modifications are summarised in Tables 1-3, below.
TABLE 1Additives to the active mass showing improved performanceAdditiveRefs.Acetylene black [2]Ca(OH)2 [3]CdO [4]Organic polymers [5]Zn alloys[6-8]SnO or Sn(OH)2, PbO or Pb(OH)2 [9]HgO + organic binder[10]Ba(OH)2 or Sr(OH)2[11]
TABLE 2Additives to the electrolyte showing improved performanceAdditiveRefs.ZnO, (NH4)2CS[12]Carbonate salts[13]Et4NBr[14]Alkyl ethers of poly(propyleneglycol)[15]Mixtures of fatty acids salts, fatty acid esters,[16]aliphatic alcohols & hydrocarbonsFluoride salts and hydroxides[17, 18]
TABLE 3Additives to the separator showing improved performanceAdditive Refs.Carboxymethylcellulose [19]Sucrose fatty acid esters [20]Sorbitan fatty acid esters [21]
Bocharov et al., USSR SU Patent No. 1 457 760 (1992) [16], disclosed an electrolyte for a nickel zinc battery containing C10-C16 fatty acids as a mixture together with a number of long alkyl chain esters, alcohols and hydrocarbons that exhibited improved performance.
Okabe et al., in two patents described the use of fatty acid esters of sucrose, Jpn. Patent JP No. 07 161 376 [20], and sorbitan, Jpn. Patent JP No. 07 161 375 [21], coated on nickel zinc battery separators to inhibit growth of dendrites through this component. It is most probable that in this strongly alkaline electrolyte, hydrolysis of the fatty acid esters occurs to yield the free fatty acids in the electrolyte.
Whilst these modifications were shown to improve the performance of nickel-zinc batteries at low charge and discharge rates, there is still a need for a rechargeable nickel-zinc battery with demonstrably low shape change together with the absence of dendrite growth at high charge and discharge rates.
It is therefore an object of the present invention to provide a rechargeable zinc electrode and rechargeable zinc cell which go some way towards achieving this desideratum, or to at least provide the public with a useful choice.